wil6210: guarantee safe access to rx descriptors shared memory

add memory barrier after allocating new rx descriptors, before
updating the hwtail.
This will guarantee that all writes to descriptors (shared memory)
are done before committing them to HW.

Change-Id: If7c91c31b490b0762d23df21db7c30652b0817d4
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Git-commit: ab6d7cc3
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/kvalo/ath.git


CRs-Fixed: 1015627
Signed-off-by: Maya Erez <merez@codeaurora.org>